In the printing arts, particularly lithography, where multicolor printing is employed, it is necessary, prior to any extended printing run, to ensure the quality of the color separation negatives, or diapositives, from which the printing plates are prepared. While "proof press" practice remains significantly widespread, there has been an increase in the use of "color-proofing" materials and procedures. Contrary to the proof press methods which entail the preparation of actual printing plates for the generation of ink impression prints, the color-proofing materials provide means for the intermediate representation of the color separations which are in great measure sufficiently faithful to the separation photography to serve as indicators of the final printing impressions which might be expected from a printing run.
The more successful color-proofing materials have generally been of two types, comprising either sets of precolored or pigmented photosensitive sheets which are exposed to the set of color separation originals and processed to provide a series of colored proof images which superimposed yield a multicolor representation of the final work, or photosensitive sheet materials which are substantially colorless yet respond to photographic exposure to yield latent images which may be colored respectively with the primary hues in the form of pigments, inks, or dyes.
The former of these two types of color-proofing materials, i.e., the precolored sheets, are generally deemed less advantageous in view of the fact that the depth or intensity of color cannot readily be varied. The requirement for a large inventory of variously colored sheet materials further detracts from their utility. For these reasons it has been recognized that the proofing materials which can be employed in a post-coloring process are better adapted to the lithographer's needs in providing preliminary reproductions of the color separation originals in those tones and variations of color intensity which will be required in the final printing. The color-proofing materials and method of the present invention belong to this latter group of post-colored proofing materials.
Presently available color-proofing materials comprise for the most part a carrier sheet and a layer coated thereon of a photographic material which reacts to light exposure in such a manner that it exhibits a greater or lesser degree of solubility in developing fluids or response to other processing as between the exposed and unexposed areas. Such photographically reactive layers are generally photoresist or photopolymerizable compositions. The photographic layer of the present invention may be generally classed with the photoresist materials.
Typical of the precolored proofing materials are those positive-working napthoquinone diazide compositions of U.S. Pat. No. 3,326,682 and negative-working benzoquinone diazide photoresist compositions described in U.S. Pat. No. 3,527,604. The color-proofing sheets utilizing these compositions generally comprise a layer of the photoresist material in which is incorporated one of the pirmary color pigments utilized in the process inks of the lithographic trade. Image-wise light exposure of the layer to a color separation original results in a photolytic change in the composition of the layer such that the exposed areas of a positive-working material, for example, become more soluble in a developing fluid, such as an aqueous alkaline solution. Washing the exposed material with such a fluid removes the light-struck areas causing the remaining colored layer material to form a faithful reproduction of the color separation original. Where the carrier for each of the precolored photoresist layers is a transparent film, superposition of each of the processed color sheets yields a composite which is representative of the final print.
Other procedures for manipulating the precolored photoresist layer proofing materials have been devised wherein the assemblage of separate sheets is avoided. These include the sequential transfer of separate photosensitive colored layers from temporary carrier sheets to a receptor upon which each such layer is exposed and processed to remove soluble portions, as described in U.S. Pat. Nos. 3,671,236 and 3,728,120. Further methods, described in U.S. Pat. Nos. 3,721,557 and 3,764,318, entail the exposure and development of each color separation print on its respective carrier prior to transfer of the resulting color images to a final receiving sheet, in sequence and proper registry with each of the other proof images of the set.
Among the post-colored proofing materials which, as earlier noted, provide the lithographer with more latitude in the preparation of proofs of varying color intensity and hue are those described in U.S. Pat. No. 3,060,024 where a photopolymer exposed to render light struck areas less tacky under applied heat is developed by the application of a finely-divided process pigment, the pigmented image being transferred with heat to a receptor sheet in register with other prints of the set, and in U.S. Pat. No. 3,634,087 where a diazo resin layer, exposed to a color separation original, is developed with an alkaline fluid to remove unexposed areas and yield an image which is colored with an ink/lacquer emulsion, the imaged proof sheet then being superimposed in registry upon other proof sheets of the set.
The color-proofing materials of the present invention, in providing for the post-coloring of exposed sheet material, avoid the noted disadvantages of those precolored or predyed sheets which have been in prior use. Further, the present materials provide the lithographer with latitude in the preparation of images of varying color intensity, and still further, provide for precisely registered compositing of multi-color images on a single sheet, thereby avoiding the excessive thickness and registry problems associated with separate sheet overlays. The present material and process also exhibit the notable advantage of enabling the preparation of composite transparency images, as well as surprint images on opaque papers and sheet materials of any stock upon which the final lithograph is intended to be imprinted.